The invention generally relates to a method of groundwater remediation and more particularly to passive remediation of DNAPL's (ie., dense non-aqueous phase liquids) from groundwater remediation wells.
Chlorinated solvents have been used widely in industry in the past, and their disposal has been historically abused. For instance, cast out in lagoons and the like, chlorinated solvents have leeched into the soil to become common contaminants in groundwater. Chlorinated solvents are included in a family widely known as dense non-aqueous phase liquids, which is commonly abbreviated to the acronym "DNAPL's". Common chlorinated solvents frequently detected as DNAPL's in groundwater include trichloroethylene (TCE), tetrachloroethylene (PCE), and 1,1,1-trichloroethane (TCA).
DNAPL's are characterized by specific gravities higher than water but viscosities lower than water. They are generally insoluble in as well as generally immiscible with water. Since DNAPL's are heavier than water, they tend to sink through the groundwater layer to the lowest level of the water bearing unit. Although DNAPL's are generally immiscible with groundwater, the solubilities of most DNAPL's in groundwater exceeds drinking water standards.
The EPA has recognized that the cleanup of DNAPL contaminated groundwater may be difficult if not impossible. As DNAPL's are heavier than water, they sink downward through our aquifers in vertical stringers, displacing water, filling pore spaces, and coming to rest in depressions in impervious layers and the like. Also, inclined bedding planes of sedimentary rock or other kinds of sub-surface heterogeneity can cause lateral spreading of the DNAPL contaminant to reach across a wide area. The standard treatment technology of "pump and treat" from remediation wells does not often work effectively. Remote pockets or pores filled with DNAPL's dissolve at a snail's pace as nearby remediation wells persistently draw up what little DNAPL seeps into the well. These residual pockets of DNAPL can act as a source of ground water contamination for many years, in fact in some cases, studies find that remediation by "pump and treat" methods would take decades or longer. Whereas studies have found that the theoretical time required for restoring DNAPL-contaminated aquifers contained in sand and/or silt deposits resting on shales, siltstones, or sandstones might be achieved in weeks:--the same studies have also found that to achieve like results in limestone and/or dolomite bedrock layers would take up to several years. Accordingly, the EPA has recently updated guidance documents which recognize that restoration of DNAPL-contaminated aquifers to Maximum Contaminated Limits (M.C.L.'s) may not be practicable everywhere.
Indeed, wherever restoration of a DNAPL-contaminated aquifer has been successfully achieved, it has been at a site where geologic conditions were especially favorable. For instance, at a site in New Jersey, DNAPL's were pooled in unconsolidated sand and silt materials on top of a clay surface. A series of recovery or remediation wells, drilled down to the pool, were able to extract by "pump and treat" technology 94% of the pooled DNAPL. See, eg., A. Michalski et al., "A Field Study of Enhanced Recovery of DNAPL Pooled Below the Water Table," GWMR, 90-100, Winter 1995. At Hill Air Force Base in Utah, a "pump and treat" process combined with surfactant flooding successfully recovered more than 23,000 gallons of DNAPL from two pools. There, the two pools were located in depressions of an impervious surface. See, eg., Oolman et al., "DNAPL Flow Behavior in a Contaminated Aquifer: Evaluation of Field Data," GWMR, 125-37, Fall 1995.
These successful DNAPL clean ups, as well as other instances reported to date, enjoyed favorable geologic conditions for removal of DNAPL's. The favorable geologic conditions included permeable unconsolidated materials underlain by an impervious layer, and that the DNAPL's were found pooled within depressions or pockets in the impervious layer. In above-mentioned reports it was also noted, however, that it was not generally possible to speed-up the recovery of residual DNAPL by increasing pumping rates. In other words, the stuff is molasses thick and dissolves only but very slowly.
Even where geologic conditions are favorable, the sinking of remediation wells and flooding thereof with surfactants also risks worsening the DNAPL contamination and spreading its extent. For instance, the drilling of monitoring and/or remediation wells through DNAPL-contaminated aquifers risks creating new artificial vertical pathways through impervious layers and hence allow additional downward sinking of the DNAPL into deeper zones of the aquifer. And to worsen a situation gone bad, the use of surfactants where vertical pathways are undefined or uncontrolled, particularly where low permeability layers had stopped the DNAPL, could result in solvent-aided penetration of the DNAPL down through the low permeability layer.
Experience with a site in Southwest Missouri has provided the incentive look for solutions to DNAPL clean up beyond the standard "pump and treat" practices of the prior art.
In particular, this site is located in bedrock comprising layers of limestone (a rock whose major constituent is the mineral calcite, eg., CaCO.sub.3) and dolomite ("dolomite" being both a rock and a mineral, the rock's major constituents being both calcite and the mineral dolomite, eg., CaMg(CO.sub.3)). More significantly, this bedrock of limestone and dolomite and the like contained classic karst solution features. The experience with a "pump and treat" process was as follows. Only fractional amounts of the estimated DNAPL load were recoverable, and at slow rates. Most of the DNAPL that could be recovered from an original well was recovered during the first 2 years of an 8-year recovery period. However, it was possible to extract a trickle of DNAPL for the remaining 6 years, with no indication at the end of the 8-year recovery that the trickle of DNAPL would not persist indefinitely. It became a practice to drill new wells adjacent to the original wells after the original wells effectively went to being non-producing after 2 years. In some cases there seemed to be local relief of the trickle condition and DNAPL could be drawn out of the new well at enhanced rates. However, wherever this was successful with a new well adjacent an original well, DNAPL recovery dropped off much sharper than had occurred with an original well, in fact the drop off occurred substantially within a period of weeks to months.
Studies were conducted to find what problems may be existing. Downhole video inspection disclosed the following matters of DNAPL's transport phenomena. DNAPL's seeped in to the wells at several elevations, generally at the bedding planes between the bedrock layers in which there might be several feet of stratigraphic intervals between deeper bedrock layers. In at least one instance, a DNAPL string appeared to seep in at one level, sink down in the well hole, and migrate out of the well at another lower bedding plane. Hence a pump at the bottom of the well had no chance to draw in that DNAPL string.
Another finding of the study is as follows, although its telling requires use of relatively difficult, technical language. By way of background, any given DNAPL composition typically includes both dense and light, insoluble and moderately soluble, volatile and non-volatile liquid and solid chemical components. Bench studies show that exposing a DNAPL to fresh water results in dissolution of the more soluble/volatile chemical phases. Likewise, in a well hole, exposing a DNAPL to large quantities of groundwater in the open well hole results in dissolution of more soluble/volatile chemical phases. As a result, this increases the DNAPL's viscosity, interfacial tension, and density. In some cases, the effect of these changes on the DNAPL entering the well hole appears to cause a "clotting" action. Put differently, the DNAPL "freezes" up the pores that previously had conducted the DNAPL into the well hole. The DNAPL thus clots the pores and backs up behind the clots at the bedrock pore/water interface. DNAPL entry at that pore and multiple others where clotting takes place, is no longer effectively possible.
Another finding of the study was much more basic. It was suspected that the majority of the DNAPL would be pooled within solution cavities in the upper bedrock zone. The experience of the drillers was substantially different. Very few DNAPL-contaminated solution cavities were encountered despite in some areas high-intensity drilling. However, in karst solution features where solution cavities are amply abundant, this experience could be explained as an example of "they are hard to hit" (eg., the "they" being DNAPL-contaminated solution cavities as opposed to the general population of solution cavities).
It became evident that a different approach would be needed for cleaning up DNAPL contamination in this limestone/dolomite bedrock containing karst solution features. What is needed is an improvement which overcomes the shortcomings of the prior art when applied to a bedrock geology having karst solution features. A number of additional features and objects will be apparent in connection with the following discussion of preferred embodiments and examples.